1. Field of the Invention
The present invention relates to a processing method for antenna patterns in an antenna apparatus of monopulse power feed system, more particularly to a two-dimensional beam compression processing method which compresses antenna patterns two-dimensionally, and relates to an antenna pattern processing method which can compress the beam width of the antenna patterns two-dimensionally and reduces side lobes.
2. Description of the Related Art
Generally, the beam width and side lobes act as indices to represent the efficiency of antenna patterns, and the smaller the beam width is or the smaller the side lobe is, the better the performance of the antenna for transmitting as well as for receiving.
However, there is an opposed relationship between the beam width and the side lobe, and the beam width is inversely to the size (length) of antenna. That is, when the size of the antenna is constant, the side lobe will be made larger if the beam width is made smaller, and the beam width will be made larger if the side lobe is made smaller. When the size of antenna may be varied, the antenna may be made larger if the beam width is made smaller, and the beam width will be made larger if the antenna is made smaller.
Therefore, for example, in a radar antenna, in the relationship between beam width side lobe, resolution capability may become worse, identification capability for objects may decline and many objects might be mistaken for one object, whenever the beam width is widened and the side lobe is necessarily made smaller. Conversely, when the beam width is made smaller, the side lobe is made large, therefore, when there is an object in the direction of the side lobe, a wrong judgement may be made that there is an object in the direction for observation. Moreover, in the relationship of the beam width and the antenna size when the beam width id reduced by half and the identification capability is doubled, the size of antenna must necessarily be doubled. If the size of the antenna is doubled not only the occupied region of the antenna is made larger, but the weight of antenna also increases as well as the antenna support structure. Conversely, when the size of the antenna is reduced by half, the beam width is doubled, and the identification capability is reduced by half.
Thus, it is impossible to optimize both the beam width and side lobe characteristics so the beam width and the side lobe are compromised to a certain degree, considering a distribution for minimizing the beam width under a condition of a certain side lobe, or for minimizing the side lobe under that of a certain beam width, such as a Chebyshev distribution. The beam width and the size of antenna having an opposite nature as described, most practical antennas have restrictions such as the region occupied by antennas, the beam width is compromised to a certain degree in the actual situation.
In order to solve these problems conventionally, a beam compression method is well-known for narrowing the beam width by subtracting a difference signal pattern of each received signal from a sum signal pattern of each received signal from the same two antennas of a monopulse power feed system. FIG. 1 shows an antenna apparatus for performing such a beam compression. Antennas 101 of monopulse power feed system with length a and distance d between their respective centers, a hybrid circuit (HYB) 102 for forming a sum signal .SIGMA. and a difference signal .DELTA. for each received signal in the two antennas 101, 101, detection circuits 103 for detecting the sum signal .SIGMA. and the difference signal .DELTA., and subtracting circuit 104 for outputting an antenna output signal after subtracting the difference signal .DELTA. from the sum signal .SIGMA..
FIG. 2 and FIG. 3 illustrate the result of a simulation when horn antennas with each of their lengths set about 25.7 times as long as the received wavelength and the distance between their centers set the same value as length a are used as the antennas 101, and the figures illustrate field patterns (FIG. 2) and power patterns (FIG. 3) normalized on the basis of the value in the direction where the angle is at zero degree, while the aperture surface distribution of the antennas is assumed to be a uniform distribution. In these figures for patterns, broken lines show the sum signal .SIGMA. and solid lines show output patterns; from these patterns, it is understood that beam compression is performed.
According to the conventional beam compression method as described above, the sum signal patterns of the original antenna patterns are given the beam compression, along with however, large side lobes generated in output patterns, as shown in FIG. 2 and FIG. 3. That is, in a near point where the value of the sum signal .SIGMA. takes zero for the first time, the value of the difference signal .DELTA. shows a maximum value, then the value subtracted the value of the difference signal .DELTA. from that of the sum signal .SIGMA. is negative as shown in FIG. 2, but the value b is much larger than the size c of the side lobe of the sum signal pattern. Therefore, in some conventional beam compression methods, a side lobe is enlarged in the point where the difference signal .DELTA. is larger than the sum signal .SIGMA., consequently, although the beam compression can be made, the side lobe will be larger than desired.
Moreover, in the conventional method for performing beam compression, only one antenna system comprises with the same two antennas of the monopulse power feed system, and the method is only allowed beam width to be compressed one-dimensionally, and its identification capability is not sufficient. That is, for example, if a main beam pattern when beam compression is not performed is a pattern 201 as shown in FIG. 4A, it is impossible to identify the objects a, b, c, d and e, in this case. If this pattern is one-dimensionally compressed toward the direction X its main beam pattern 202 is given as shown in FIG. 4B, and the identification capability is improved compared to the case which beam compression is not performed. However, the identification capability is not sufficient, because it is still impossible to identify the objects a, c and e.